It has been difficult to disperse carbon nanofibers (filler) in a matrix. The inventors proposed a method of producing a carbon fiber composite material that uniformly disperses carbon nanofibers in an elastomer by improving the dispersibility of the carbon nanofibers (see JP-A-2005-97525, for example). According to this method, the elastomer and the carbon nanofibers are mixed, so that the dispersibility of the carbon nanofibers with strong aggregating properties is improved due to a shear force. Specifically, when mixing the elastomer and the carbon nanofibers, the viscous elastomer enters the space between the carbon nanofibers while specific portions of the elastomer are bonded to highly active sites of the carbon nanofibers through chemical interaction. When a high shear force is applied to the mixture of the carbon nanofibers and the elastomer having an appropriately long molecular length and a high molecular mobility (exhibiting elasticity), the carbon nanofibers move along with the deformation of the elastomer. The aggregated carbon nanofibers are separated by the restoring force of the elastomer due to its elasticity, and become dispersed in the elastomer. Expensive carbon nanofibers can be efficiently utilized as a filler for a composite material by thus improving the dispersibility of the carbon nanofibers in the matrix.
Carbon nanofibers are industrially mass-produced by a vapor growth method that pyrolyzes a gas such as a hydrocarbon gas in the presence of a metal catalyst. Carbon nanofibers which are mass-produced by the vapor growth method in a heating furnace at about 1000° C. and further graphitized by heating at a higher temperature have been known (see JP-A-2006-198393, for example). The carbon nanofibers thus graphitized have a surface with a small number of defects. However, such carbon nanofibers exhibit poor wettability with matrix material (e.g., elastomer).